It is found that there is an upper-limit critical power for self-guided propagation of intense lasers in plasma in addition to the well-known lower-limit critical power set by the relativistic effect.Above this upper-limit critical power,the laser pulse experiences defocusing due to expulsion of local plasma electrons by the transverse ponderomotive force.Associated with the upper-limit power,a lower-limit critical plasma density is also found for a given laser spot size,below which self-focusing does not occur for any laser power.Both the upper-limit power and the lower-limit density are derived theoretically and verified by two-dimensional particle-in-cell simulations.The present study provides new guidance for experimental designs,where self-guided propagation of lasers is essential.
The photoelectron momentum distribution of H＋ in circularly polarized laser fields is studied based on classical trajectory calculations. We screen Coulomb potentials at different radii, and trace trajectories of an ensemble of electrons in such screened Coulomb potentials and circularly polarized laser fields. Simulations show that electron trajectories are bent by Coulomb fields, resulting in the laser-intensity-dependent drift of photoelectron momentum distributions in the laser polarization plane. This study intuitively explains how Coulomb potentials modify photoelectron momenta.
实验上使用大能量、亚ps激光脉冲大角度入射固体靶,获得了沿靶面方向定向传播、发散角仅有2°、峰值能量为3–4 Me V的准直、准单能电子束.实验发现激光对比度对靶面电子束的产生起到了至关重要的作用,最佳的对比度为5×10-6.在此最优化条件下,通过背向散射光谱分析发现,共振吸收激发的等离子体波加速可能是电子的主要加速机制.探针光阴影成像及等离子体自发光的精细结构显示,预脉冲与固体靶相互作用中产生了尺度100μm左右的过临界密度预等离子体.这种等离子体的作用类似于等离子体反射镜,使得激光脉冲被限制在预等离子体区与靶面之间,因而最终造成了电子束沿靶面方向的导引.这种靶面电子束因其合适的能量范围、高度的准直性及沿靶面方向定向传播的特性有望在惯性约束聚变尤其是锥靶快点火中得到应用.
Terahertz （THz） radiation has attracted much attention due to its wide potential applications. Though radiation can be generated with various ways, it is still a big challenge to obtain strong tabletop sources. Plasma, with the advantage of no damage limit, is a promising medium to generate strong THz radiation. This review reports recent advances on strong THz radiation generation from low-density gases and high-density solid targets at different laser intensities.
At Shanghai Jiao Tong University（SJTU） we have established a research laboratory for advanced acceleration research based on high-power lasers and plasma technologies. In a primary experiment based on the laser wakefield acceleration（LWFA） scheme, multi-hundred Me V electron beams of reasonable quality are generated using 20–40 TW, 30 femtosecond laser pulses interacting independently with helium, neon, nitrogen and argon gas jet targets. The laser-plasma interaction conditions are optimized for stabilizing the electron beam generation from each type of gas. The electron beam pointing angle stability and divergence angle as well as the energy spectra from each gas jet are measured and compared.
Ahmed M.M.ElsiedNasr A.M.Hafz李松Mohammad MirzaieThomas Sokollik张杰
